Refining of crude methanol by azeotropic distillation



2 sheets-sheet 1 July 12, 1949- J. F. MccANTs l REFINING 0F CRUDE METHANOL BY AZEOTROPIC DISTILLATION vFiled Aug. 4, 1945 Patented July 12, 1949 REFINING oF canna ME'rnANoL BY AzEo'raoPrc Dls'nLLA'rIoN of Delaware .lames F.iMccants, Pawhuska, Okla., assignor to Skelly Oil Company, Tulsa, Okla., a corporation Application August 4, 1945, Serial No. 608,970

Claims.

The present inventionrelates to the reilningfof crude methanol containing'- small amounts of empyreumatic materials that cannot be completely separated from the methanol .by ordinary fractional distillation.

Crudemethanol produced by any oi' numerous prior' art methods usually contains impurities, some of lwhich impart an offensive odor to the product* TheV impurities which are-responsible.-

yfor, imparting the oiensive yodor -arepresent in very low concentration and` are very difficult structurally to identify. The generic term "empyreumaticv impurities will .be used herein to refer to these odor imparting materials.

l A. primary object of this invention is to provide a new and useful process for treating crude methanol to produce a product essentially free vfromsuch empyreumatic materials .as are originally contained therein. v y

A further object is to provide a method for reiining and' purifying the methanol fractions of the crude liquid product; producedfvbyy certain methods, such .as the vapor phase partial oxidation of gaseous hydrocarbons.

One feature of the processcomprises fraction-l ally distilling crude methanol containing empyv reumatic, materials in` the presence ofy'a norma1lyliquid non-aromatic hydrocarbon which forms a minimum boiling azeotropic mixture with methanol whichpasses out `of the-,distillation zone as an overhead productthat, upon cooling, condenses. and forms two phases. In accordance with this invention,this process yis-applicable to crude methanol `fractions ,having rela tively wide boiling ranges. The preferred method .for carrying out this process involves fractional distillation of crude methanol in the presence of two different types of liquids, one a normally-liquid aromatic hydrocarbon and the other .a normally liquid aliphatic or alicyclic hydrocarbon. f Y y, s z

In carrying out the preferred method of this. process, it is essential that the aromatic hydroor alicyclic hydrocarbon-methanol azeotrop, and furthermore that the aromatic hydrocarbon be charged inexcess of that amount necessary for aromatic hydrocarbon-methanol azeotrope'formation. The reasonfor this boiling temperature differential and use of excess aromatick hydrocarbonl will become more` evident upon further reading of the herein described process.

y I have further discovered thatv certain derivatives of hydrocarbons, such as' the halogenated hydrocarbons, may be substituted for' the norcarbon-methanol azeotrope boil somewhat above the boiling point of the normally liquid aliphatic mally liquid aromatic hydrocarbonsin themanner herein described. Examples of 'such halogenated hydrocarbons arefamyl chloride, 1,2,di-

'chloro-norr'xial-butane, chloroform, and carbon tetrachloride.

The empyreumatic impurities inlowmolecu# .lar weight crude alcohols are more soluble in' the normally liquid aromatic or halogenated hydrocarbons than theyA are in the normally yliquid vvaliphatic or -alicyclic hydrocarbons.

s With this fact in mind, my discovery utilizes ,the fact that the yaromatic or halogenated hydrocarbons exercise a selective solvent action upon the empyreumatic impurities, thus leading to the greatly improvedy s process vof separating these from the methanol, which process is` the subjectI ofthe present invention. p. .a f v- A novel feature of my invention consists in adding an aromatic hydrocarbontoan aliphatic .or alicyclic hydrocarbon and crude methanol and distillingthe resulting. mixture in a fractionating column. By virtue of the respective boiling points, the aliphatic or lalicyclic,hydrocarbon- `methanol azeotrope formed-is taken ofil as'the overhead product, while the aromatic hydrocarbon-methanol azeotropev is retained in the .column, and the excess aromatic hydrocarbon and empyreumaticimpurities are vWithdrawn as .the kettle product. Thearomatic hydrocarbonmay be separated from the empyreumatic impurities and suiiiciently purified for further use by agitating and extracting with water, since the empy-v reumatic impurities, being quite solublein water,

are easily washed from the aromatic hydroprovement in the purification process .by the addition of the aromatic hydrocarbon is not known, it is believed that such an improvement is due to two factors:

(1) The selective solvent action of the excess' hydrocarbon azeotrope which, by virtue of itshigher boiling point, is continually retluxed back down the column, carrying with it the empyreumatic impurities.

Another feature in my present invention resides in condensing and cooling the overhead distillate from the fractionating column to a sumcient temperature, and without adding water or diluting it in any other way, to obtain phase separation. That is, in this process, I separate and distill directly, and without intermediate aqueous dilution, the alcohol-rich lower layer from the reflux' separator, this being done in a second distillation column. One aspect of thev invention, therefore, consists in providing an improved process of separating, purifying, and refining crude methanol, which is more particularly described hereinafter.

A crude oxidation product, produced by the vapor phase partial oxidation of gaseous aliphatic hydrocarbons, comprises an aqueous solution of a large number of compounds including acetaldehyde, acetone, acetals, methanol, ethanol, isopropyl and propyl alcohols, formaldehyde, organic acids, etc. By ordinary fractionation this crude liquid oxidation product can be separated into a fraction containing acetaldehyde, a second fraction containing methyl acetone, a third fraction comprising chiey methyl alcohol, a fourth fraction comprising chiefly higher alcohols, and a residual fraction ofcrude aqueous formaldehyde.

The third fraction comprising chiefly crude methanol is preferably purified of certain aldehyde impurities -by polymerizing with caustic soda, which step is accomplished by heating the crude methanol in a batch still withsufiicient caustic soda to polymerize any aldehyde impurities present, and'to drive 0E as overhead vapors partially puriiied methanol, leaving as a still residue the waste caustic and aldehyde polymers.

j It may be advantageous to combine this purifica.-v

tion treatment with the fractional distillation step used in isolating a crude methanolfraction from the remainder of the oxidation mixture.

Irrespective of the care with which the aforementioned caustic treatment and fractionation is carried out, this type of partial purification does not remove all of the empyreumatic impurities which impart an offensive and pungent odor to the methanol. For testing purposes, it.

is convenient to determine the total amount of said empyreumatic impurities, expressing the amount of said impurities in terms of apparent dimethyl acetal concentration. This method involves treating a sample of methanolcontaining said impurities with an equal volume of concentrated sulfuric acid and matching the color, produced with methanol solutions of dimethyl acetal of knownV concentration when treated in like manner with concentrated sulfuric acid.

Fig.l 1 is a flow sheet illustrating the procedure and arrangement of equipment ilor a semi-continuous or batch azeotropic distillation according to the process of the present invention;

Fig. 2 is a flow sheet illustrating the procedure and arrangement of equipment for continuous 'azeotropic distillation` employing the method of the present invention.

In the diagrams and inthe descriptions of the following operations, no attempt has been made to indicate the position of auxiliary apparatus such as heat exchangers, heating sources, condensers, pumps and other pertinent equipment, as the proper placement of these will at once be apparent to those skilled in the art.

In the following descriptions the process is described and claimed as if carried out at atmospheric pressure, which at the place where Athe: process was carried out is approximately '140 millimeters. It will be understood, however, that subatmospheric pressures or superatmospheric pressures may be used without departing from the spirit and scope of the invention.

The following examples and table illustrate the principles underlying my process for refining and purifying crude methanol as applied in actual operation:

EXAMPLE I Reference is made to Fig. l. This shows a. semi-continuous or batch operation, using as a charging stock 300 m1. of caustic-treated methanol, containing 1.25 per cent of apparent dimethyl acetal as an impurity. This was charged through pipe 2 to kettle 4 of fractlonating column B. To this fractionator, through pipe 2, was also charged 200 ml. of commercial grade heptane along with 1D0 ml. of toluene. Heat was applied to the kettle 4 at the base of the column. When the temperature at the top of the column reached about 59 C., distillate vapors composed of heptane and methanol in azeotropic proportions 4were vapors were condensed and cooled to about 25 C.

in the condenser I0. On cooling to this temperature the condensate formed a methanol-rich phase and a heptane-rich phase, and the mixture of phases being passed to separator I4 at the approximate vertical center thereof through pipe I2, where the methanol-rich phase settled to form a lower layer and the heptane-rich phase rose to form -an upper layer. 'Ihe heptane-rich upper layer was withdrawn through pipe I6 and returned to the upper portion of fractionator 8 as reux. The methanol-rich lower layer was withdrawn through pipe 20 and introduced into a kettle 22 at the base of fractionating column 24 constituting a second distillation zone for other treatment, as hereinafter described. The material remaining in fractionating column 6 and kettle 4 formed a second azeotroplc mixture comprising methanol and toluene, having a boiling point of about 63 C. lThe empyreumatie impurities contained in the crude methanol, being more soluble in toluene than in heptane, were concentrated in kettle 4 and in the lower plates of column 6 and were withdrawn as a part of the residual fraction of toluene and excess heptane from kettle 4 through pipe IB.

Heat was supplied to kettle 22 to which the methanol-rich layer from reflux separator i4 was discharged. The mixture of the' methanol-rich layer consisted predominantly of methanol and a small amount of heptane. When the temperarture at the top of fractionating column 24 reached about 59? C., distillate vapors composed of heptane and methanol in azeotropic proporvaporized and returned from reboiler 44 to column tions were withdrawn through overhead .vapor pipe 28, leaving as a kettle product su-lzistantlallyv pure methanol, which was Withdrawn from kettle 22 through pipe 32. The overhead vapors ywere condensed and cooled t-o about 25 C. in condenser 28. On cooling to this temperature the condensate formed a methanol-rich phase and a heptane-rich phase, which mixture of phases was passed through pipe 30 to separator I4 at the vertical center thereof where the methanol-rich phase settled to form a lower layer and the heptane-rich phase rose to form an upper layer. These layers self-evidently blended lwith similar layers already present.

The apparent dimethyl acetal impurity content of the methanol treated in the method described above was reduced from 1.25 per cent to 0.075 4per cent. crude methanol charging stock, having a dimethyl acetal impurity of 1.25 per cent was processed in this batch distillation without employing the second azeotropic agent, i. e., the toluene. The apparent dimethyl acetal impurity content of the methanol thus treated without the azeotroplc agent toluene was reduced from 1.25 per cent to 0.20 percent, thus showing the beneticial elects of the added aromatic azeotropic agent in purifying methanol-containing empyreurnatic impurities.

EXAMPLE II Reference is made to Fig. 2. This Shows a :ontinuous operation, using as a charging stock t fraction of crude methanol in admixture with tn aromatic hydrocarbon, in this example benzene, which was charged to fractionating column l through pipe 4i. An aliphatic hydrocarbon, n this instance, hexane (boiling Arange i771 C.), was charged to fractionating column L0 through pipes 42 and 43, in suicient quantity o start the system in operation. Heat for this :olumn was provided by a heater or reboiler 44 t its base, into which excess benzene and empyeumatic impurities from the base of the column lvere withdrawn vthrough pipe 45 to be heated ..nd returned asvapors to the column through |ipe 4B. In iractionating column 40 there were ormed two azeotropes; one was a hexane-methvvno1 azeotrope (boiling point about 50 0.); and he other was a benzene-methanol azeotrope. boiling point about 58 C.). When thetemperaure at the top ofthe column 40 reached approxinate1y 50 C., distillate vapors, composed of iexaneand methanol, were continuously with- ,rawn from said column through overhead pipe ll,- and were condensed and cooled to approxilately .25," C.v in -condenser .48. On cooling to his temperature, the condensate formed a hex- `ne-rich phaseand a methanol-rich phase. The iixture of phaseswas passed to reflux separator 9 at the approximate vertical center thereof, hrough pipe 50, where the methanol-rich phase ettled to form alower layer and the hexane- A second portion of the same 40 through pipe 46, was continuously withdrawn through pipe 53 and mixed with water introduced through pipe 54, and then passed to separator 55, in which two phases formed. The lower phase comprised water and empyreumatic vimpurities and the upper phase comprised substantially pure benzene. The purified benzene was continuously returned through pipes 56 and 4I for reuseln column 40, while the water and empyreumatic impurities were continuously withdrawn as a residue fromthe separator 55 through pipe 51.

The second fractionating column 52 to which the methanol-rich layer from reflux separator 49 was discharged, was supplied with heat from' a reboiler 58 to which substantially pure methanol from the base of fractionator 52 was introduced by pipe 59. Methanol vapors generated in reboiler 58 werel returned to column 52 through pipe 60. In column 52, the introduced methanolcomprising an azeotroplc mixture of hexane and methanol .and a residual productof substantlally' pure methanol, which was continuously withdrawn from the reboiler through the product discharge pipe 8l. The hexane-methanol azeotropic vapors left the top o f the fractionating column 52 at a temperature of about 50 C.

'through pipe 63 and were condensed and cooled in condenser 64 to approximately 25 C., and thenpassed to reflux separator .48 through pipe 65 at the approximate vertical center thereof where phase separation occurred as above described.

Ina modified form of eitherof the examples given, some portion vof the methanol-rich layer from the reflux separator might be returned with the aliphatic hydrocarbon layer to the fractionating columns 6 or 40 as reflux.

While the present invention has been described, so 'far as, these specilic examples oi' methods of semi-continuous and continuous operations are concerned, it is to be understood that aliphatic hydrocarbons other than heptane and hexane may also be used. Pentane, for example, formswith methanol a binary azeotrop'e boiling at about 31 C., which is very eiective for purifying methanol. The percentage of 4methanol in the methanol-hexane azeotropic mixture (Example II) is about 51 mole per cent, while that of methanolin a pentane-methanol azeotropic mixture is about 13 mole per cent. Aromatic hydrocarbons, other.` than'benzene and toluene,

lchphse rose to form an upper layer. The

.exane-ricli upper layer' was continuously withrawn through pipe 43, and returned to the upper ortionof column 40.` kThe methanol-rich lower tyer vwas continuously withdrawn through pipe' I, and introduced' at "an-intermediate level into second fractionating column 82, constituting second distillation zone for other treatment, as

ereinafter described. The 'residual product of enzene and" empyreumatlc impurities, not

such as the xylenes, may be, used as aromatic azeotropicagents to remove asubstantial amount of the impurities from the methanol; and aliphatic hydrocarbons other than heptane, hexane, and pentane, may also be used as the aliphatic azeotrope-forming agents, such asfor example the octanes, and the dimethyl pentanes. The use of halogenated hydrocarbons in' place of the aromatic hydrocarbons has already been mentioned.

To illustrate the eillcientresults obtained by aromatic hydrocarbon, it is possible to use a.

higher boiling aliphatic a'zeotropic'agent and still produce highly purified methanol.

' the purifying agents.

Illustrative of the degree of purication which it is possible to secure by this new and improved process, a narrow boiling range fraction of methanol, containing 1.25% of apparen dimethyl acetal, was puriied by one of the processes 1heretofore described, using benzene and hexane as 'I'he final ,product contained no detectable empyreumatic impurities. From the same narrow boiling fraction of crude methanol containing 2.9% of apparent `dimethyl acetal, when subjected to purification using toluene and commercial heptane as the purifying agents, a productof purified methanol containing less than 0.12% dimethyl acetal was produced.

As the boiling point of the methanol-azentropic agent mixture which is collected as the overhead product from columns 40 and 52, as shown in the Aillustrative Example II, approaches the boilingpoint oiv methanol, the tendency for the empyreumatic impurities to be carried over with the azeotropic mixture increases, thus reof forming a second, higher-boiling azeotrope with the methanol or other alcohol being purified, is capable of carrying out of the system, in substantially unvolatilized form, the empyreumatic impurities contained in the alcohol that is be- Y ing subjected to this method of purification.

ducing the effectiveness ofthe purification process. However, the higher boiling azeotropic mixtures are more economical to use because a higher percentage of methanol is contained in the overhead product. It is in this critical range that the added ell'ectiveness of the second purifying agent, namely an aromatic hydrocarbon, is of greatest value. In other words, it then becomes possible to use a higher boiling more emcient normally liquid aliphatic azeotropic agent than could be used if the aromatic compound were not present.

The process of the present invention is particularly directed to the purification of crude methanol: however, it may also be used for separating and concentrating other oxidation products, such as the aldehydes and ketones normally present in oxidation products. The process may likewise be used topurify such higher boiling alcohols as ethanol and propanol.

While the general principles of the improved purification methods have been hereinabove described in some detail, it is to be understood that the invention is by no means limited to the exact examples given, as these are given merely by means of illustration. o The essential feature is the use of the second liquid which, while capable The use of alicyclic hydrocarbons as the nonaromatic hydrocarbons cin the process described above is claimed in my application Serial No 608,971, filed August 4, 1945.

Accordingly, I claim:

1. In a process for the elimination of empyreumatic impurities from methanol by azeotropic distillation with a normally liquid non-aromatic hydrocarbon having from ve to eight carbon atoms per molecule and in which process the methanol forms an azeotropicaliy boiling mixture with said hydrocarbon that forms an overhead product of said distillation, the improvement which comprises carrying out said process in the presence of an amount of a second liquid in excess of that required to form a second azeotropically boiling mixture with said methanol, said second liquid being selected from the group consistng of the aromatic hydrocarbons benzene toluene and xylene, and of halogenated hydrocarbons selected from the group consisting of amyl chloride, 1,2-dichloro-n-butane, chloroform and carbon tetrachloride, which second liquid enables the more adequate and complete separation o1 the empyreumatic impurities from the 'methanoj in the form of a non-volatilized material capable of being withdrawn as such from the distillatior zone in association with said second-hamer liquid.`

2. The process of claim 1 in which the seconc liquid is benzene.

3. 'I'he process of claim 1 in which the seconc liquid is toluene.

4. The process of claim 1 in which the seconf liquid is xylene. 5. Process of purifying methanol which comprises conjointly distilling it through 'a fractionating system with a sufficient amount of a satu rated aliphatic hydrocarbon having vfromiive t( eight carbon atomsJ `per molecule `to form ar azeotropically boiling mixture with said metha..

no1 and an amount of an aromatic hydrocarbor selected from the group consisting'of benzene atrasos toluene and xylene in-.excess oi' that'required to form a second azeotropically boiling mixture with said methanol; separating the lower-boiling methanol-aliphatic hydrocarbon azeotropic mix-- ture as an overhead distillate vfrom the higherboiling methanol-aromatic hydrocarbon azeotropic mixture; withdrawing as a kettle product the excess aromatic hydrocarbon containing empyreumatic impurities, adding water thereto to effect the separationof aromatic hydrocarbon therefrom, and recirculating said aromatic hydrocarbon to the initial distillation zone; cooling and condensing the overhead distillate into liquid saturated aliphatic hydrocarbon-rich and methanol-rich phases. returning the saturated aliphatic hydrocarbon-rich phase to the initial distillation, and redlstilling the methanol-rich y distilling the methanol-rich phase in a second distillation zone to form purified liquid methanol and a vaporous heptane-methanol azeotrope, and cooling the latter to form a liquid methanol-rich and a liquid heptane-rich phase, and returning with said methanol into an initial distillation phase in a second distillation zone, to form a. i

purified liquid methanol and a saturated aliphatic hydrocarbon-methanol azeotrope, cooling the said azeotrope to-form a liquid methanol-rich and a liquid saturated `aliphatic hydrocarbon-rich phase, and returning the latter to the initial distillation zone. and withdrawing vthe purified liquid methanol as the kettle product. c

6. The process of claim 5 in which the nonaromatic hydrocarbon is hexane and the aromatic hydrocarbon is benzene.

7. The process of claim 5 in whlchthe nonaromatic hydrocarbon is heptane and the aromatic hydrocarbon is toluene.

8. The process of claim 5 in which the nonaromatic hydrocarbon is octane and the aromatic hydrocarbon is xylene.

9. Process of purifying methanol which comprises mixing it with suicient amounts of heptane and toluene, the former sufficient to form an azeotropically boiling mixture with the metha- -zone, whereby two different boiling azeotropes form: one comprising hexane and methanol having a boiling point of about 50 C. at atmospheric pressure, and one of benzene and methanol having a boiling point above about 58 C. at atmospheric pressure; distilling oi the rst azeotrope 'as an overhead product, cooling it to cause its separation into a hexane-rich phase and a methanol-.rich phase and returning the hexane-rich phase to the initial distillation zone; withdrawing the excess benzene and empyreumatic impurities from the kettle of the initial distillation zone. adding water thereto to eiiect the separation of benzene therefromI and recirculating said benzene to the initial distillation zone; redistilling the aforesaid methanol-rich phase in a second distillation zone to form liquid purified methanol and a vaporous hexane-methanol azeotrope, withnol and the latter in excess of that required also to form an azeotropically boiling mixture with the methanol and heating the resulting mixture of heptane, toluene and'methanol in an initial distillation zone to a temperature of about 63 C. (at

substantially atmospheric pressure) "whereby a methanol-heptane azeotrope having a boiling point of about 59 C. and a methanol-toluene azeotrope having a boiling point'about 63 C. are

formed; distilling oi! the iirst azeotrope, separating it by cooling into a liquid heptane-rich and a liquid methanol-rich phase, returningthehepdrawing the purified methanol as a kettle product, and coolingl the said vaporous azeotrope to form a liquid methanol-rich and a liquid hexanerich phase and returning the latter to the initial distillation zone.

JAMES F. MCCANTS.

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tane-rich phase to the distillation zone, x'e-.r f

Bloomer July 10, 1945 

